fix multi poly with rects

[SimonDanisch]

poly([Rect2f(0, 0, 1, 1)], color=:green, strokewidth=2, strokecolor=:black) was missing a linesegment, since it was using the scatter conversion pipeline.

[MakieBot]

Compile Times benchmark

Note, that these numbers may fluctuate on the CI servers, so take them with a grain of salt.

using time

master:  9.94 < 10.05 > 10.29, 0.13+-
pr:      10.02 < 10.15 > 10.24, 0.07+-
speedup: 0.97 < 1.00 > 1.01, 0.01+-
median:  +0.98% => invariant

This PR does not change the using time.

ttfp time

master   28.78 < 28.99 > 29.31, 0.19+-
pr       28.82 < 29.14 > 29.62, 0.28+-
speedup: 0.99 < 1.00 > 1.00, 0.01+-
median:  +0.51% => invariant

This PR does not change the ttfp time.